International Geologiical Congress - Oslo 2008

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MPC-01 General contributions to geochronology and isotope geology

 

Juvenile and old components in Proterozoic crust; examples from Lu-Hf isotopes in zircon from magmatic Svecofennian and rapakivi rocks in Sweden

 

Ulf B. Andersson, Swedish Museum of Natural History (Sweden)
William L. Griffin, Macquarie University (Australia)
Graham Begg, Minerals Targeting International PL (Australia)
Karin Hogdahl, Lund University (Sweden)
 

 

The Fennoscandian Shield contains an Archaean nucleus in NE that is succeeded by younger accreted growth zones towards the SW. The earliest of these is the vast Svecofennian Domain with 1.95-1.86 Ga outcropping early orogenic rocks. The assembly of the Svecofennian Domain is inferred to be the result of accretion of a collage of juvenile arc terranes and matured arcs/microcontinents in a series of events or orogenies that was accomplished by 1.86 Ga. This was succeeded by the continental margin magmatism of the early Transscandinavian Igneous Belt (TIB) in the west, and syn- to late orogenic magmatism within the domain (1.85-1.75 Ga). After cratonization of the Svecofennian Domain and TIB, in the time frame 1.65-1.47 Ga, A-type rapakivi complexes were emplaced during a tensional regime, possibly related to subduction/accretion in the west.

Extensive Nd isotopic data has shown that both mafic and felsic rocks of the Svecofennian Domain are essentially juvenile, yielding isotopic values between CHUR and DM. However, some granitoids tend to be slightly more enriched with TDM values up to c. 2.2 Ga. This, together with relatively abundant 2.1-1.95 Ga zircons in both metasedimentary and as xenocrysts in igneous rocks, indicate that Svecofennian crust began to form before 1.95 Ga, in the 'Proto-Svecofennian'. Additionally, the sub-Svecofennian lithospheric mantle is typically less depleted than DM, mostly 'mildly depleted' in εNd(t) as show by 1.9-1.8 Ga and younger mafic rocks. In central Sweden, rapakivi rocks have exceptionally low εNd(t) values (-10 to -5), suggesting the presence of older, Archaean basement components in this area.

New Hf isotopic data was collected on zircons from four early Svecofennian rocks (c. 1.89 Ga), two felsic volcanic and two mafic intrusive rocks, as well as one early (c. 1.85 Ga) TIB rock from Bergslagen, and five rapakivi rocks (1.53-1.50 Ga) from central Sweden. The initial εHf values of the mafic rocks range: +4.0 to +8.0, for the felsic metavolcanic rocks: -0.5 to +6.3, and for the rapakivi rocks: -8.7 to -0.5. In addition to the c. 1.85 Ga zircons (εHf (t) +2.2 to +5.7) the TIB granite contains inherited zircon between 1.97-2.03 Ga (+3.3 to +5.0), and 2.45-2.47 Ga (-12.9 to -3.1), respectively.

The data of the early Svecofennian felsic rocks closely resemble previous data from equivalent rocks as well as 1.85-1.78 Ga TIB and late Svecofennian rocks, falling in between CHUR and DM. The mafic rocks are only slightly more depleted. These data, together with inherited zircons (with juvenile εHf) and Nd data, support the presence of 2.1 Ga Proto-Svecofennian crust in Bergslagen, and a 'mildly depleted' sub-Svecofennian mantle. The εHf(t) data of the central Swedish rapakivi zircons plot between CHUR and the upper level of the Archaean crust, and mostly below the field of evolution for the juvenile Svecofennian crust. This suggests, in accordance with the Nd data, a mixed Svecofennian-Archaean provenance for these magmas.

 

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